# Simple 555 Delay

Discussion in 'The Projects Forum' started by boonxiong, Oct 17, 2011.

1. ### boonxiong Thread Starter New Member

Oct 17, 2011
10
0
Hello Everyone,
I have a very simple circuit that I hope you guys can help me with.
I want to use the 555 circuit to do the following.

Switch is press ON. Wait X amount of seconds. Then send a single pulse out.

I've looked online everywhere for schematics but it seems I may need to be explained to like a child. Can anyone help me with this?

Thanks everyone.

2. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
Welcome to AAC (AllAboutCurcuits). You've come to a very popular site with a hose of resources and highly knowledgeable members. We do enjoy helping other electronic enthusiasts but like it more when you make a point to help yourself. And you help yourself by helping us understand in more detail what you are trying to accomplish.

Let me see if I can get the basics of what you want.
1) Manually pressing of a switch(momentary I will assume)
2) A delay count is initiated
3) When time has elapsed, send a pulse signal out

So how long is the delay?
What is the pulse signal, a single pulse or continuous pulses?
What is the amplitude of the pulse?
Is it a square wave pulse?
How long does the pulse have to be on?
What will the Pulse do?
What power source will you use?

Oh yes, a basic schematic drawing is always helpful too (.PNG image format prefered)

3. ### boonxiong Thread Starter New Member

Oct 17, 2011
10
0
Hello iONic,

I would like the delay to be 5 seconds. Time does not have to be very exact. After 5 seconds, I would like a square pluse of 2 seconds. It will be just a single pluse. I will be using a 9v battery.

What I am doing is connecting the output to a reed-relay to push a button.

Is this simple to do?

4. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
Yes, it is fairly simple..

I believe this is the basic circuit that would do the trick, minus the resistor @ capacitor values. Output A would be your 5 sec. delay and Output B would be your 2 sec square wave pulse.

5. ### boonxiong Thread Starter New Member

Oct 17, 2011
10
0
what do you mean by "minus the resistor @ capacitor values."?

6. ### elec_mech Senior Member

Nov 12, 2008
1,513
193
I think iONic meant resistor and capacitor values. The circuit iONic posted shows you roughly what you'll need and how to wire it. You'll need to do some math to pick the right resistor and capacitor values to get the delays you want.

Refer to this and read up on monostable circuits:

http://www.kpsec.freeuk.com/555timer.htm

Look at the examples and explanations in the link above, refer to iONic's posted circuit, and switch out the appropriate resistor and capacitors with the values of the ones you need to achieve your delay times.

Good luck.

7. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
If you follow the formula in the circuit image I posted one possible outcome for your Resistor & Capacitor values is:

5 sec delay (R1A - 470K, C1A - 10uF)
2 sec pulse (R1B - 200K, C1B - 10uF)

8. ### boonxiong Thread Starter New Member

Oct 17, 2011
10
0
so R1A and C1A are Resistors and Capacitors that I will have to mess around with to get the right timing?

9. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
Yes. R1A/C1A reference the delay of the LM555A while R1B/C1B reference the pulse of LM555B.

So looking at the Timing Diagram above: Output A is the 5 sec delay time. When it runs out pin 3 from LM555A IC sends a low pulse to pin 2 of the LM555B IC and triggers a 2 sec pulse on pin 3.

10. ### boonxiong Thread Starter New Member

Oct 17, 2011
10
0
Hey there iONic,
Please hang on with me as I am still learning. I can't find any capacitors that are .01uf. I'm looking at my local radioshack and the smallest I find is a 1.0uf. What difference will this make?

Another question is the polarized capacitor. What is the difference between the two at .01uf and C1A+

11. ### elec_mech Senior Member

Nov 12, 2008
1,513
193
The store you're going to may not carry them or might be out. RadioShack does carry them though:

Click on Find It In Store to see which stores in your area have them in stock. Note some stores carry more parts than others - if a store has 4 columns of the parts drawers, they'll likely have what you're looking for. Some stores only have two columns indicating they don't carry a lot of parts (mostly stores within malls).

They also have 0.1uF:

Capacitor size for small capacitors in this application shouldn't make much of a difference - ceramic capacitors are generally smaller than the rest, <1uF, and used to filter out noise generated by the IC(s) and power supply. 1uF is a little high. If it is a ceramic, you're probably okay, but if it is polarized like an electrolytic or tantalum, then you should get the ceramic instead.

Polarized capacitors are often rated higher, > or = 1uF and are often used for filtering larger spikes, used for timing (such as this circuit), etc. More knowledgeable people can explain it better. Check this post out for some more info:

12. ### boonxiong Thread Starter New Member

Oct 17, 2011
10
0
Hey elec_mech,
Thanks for your input! It does helped me out. I was able to locate a close RadioShack near me that has the .01uf. I still don't get the polarized capacitor. All I have right now are the blue capacitors. Does the + sign mean that they are not ceramic?

13. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
If its blue it could very well be a tantalum cap which would have a + sign and it would be polarized.